Thrombin, a serine protease, is an essential component of the coagulation cascade. It is produced immediately in the brain after an intracerebral hemorrhage (ICH) or after the blood- brain barrier breakdown that occurs following many kinds of brain injury. Evidence, both in vivo and in vitro, indicates that high concentrations of thrombin within the brain parenchyma can be deleterious. However, low concentrations of thrombin are neuroprotective. Thus, we have demonstrated that intracerebral pretreatment with a low dose of thrombin (thrombin preconditioning, TPC) reduces brain injury induced by ICH, cerebral ischemia and Parkinson's disease. In order to elucidate the cellular mechanisms involved in TPC-induced neuroprotection we have devised a new rat ICH model. This employs intrahippocampal injection of blood (or blood component). This has the advantage of causing easily quantifiable neuronal loss in contrast to the intracaudate model of ICH in the rat which causes more diffuse injury. Work from our laboratory also suggests that protein synthesis (e.g. iron-handling proteins and heat shock proteins) is essential in thrombin-induced neuroprotection. In this proposal, we will examine the role of thrombin, protease-activated receptor-1 (PAR-1, a thrombin receptor) and the activation of phosphatidylinositol 3 kinase (PI3K)/ ribosomal protein S6 kinases (p70 S6K) and extracellular signal-regulated kinases (ERK)/p70 S6K pathways in mediating protection from neuronal death following ICH. Phosphorylation of p70 S6K is a crucial regulator of protein synthesis. We will test the following two specific aims. 1. To determine whether or not thrombin preconditioning reduces ICH-induced neuronal death through PAR-1 activation. 2. To determine whether or not thrombin-induced neuroprotection results from activation of the PI3K/p70 S6K and ERK/p70 S6K pathways. PUBLIC HEALTH RELEVANCE: The purpose of our project is to investigate the mechanisms involved in TPC. Thrombin- induced brain protection seems to be PAR-1 mediated, greatly facilitating analysis compared to ischemic preconditioning. The long term goal of these studies is to find mechanisms that can be used to limit brain injury after ICH. At low concentrations, thrombin protects the brain. Researching how to mediate thrombin will allow us to create new ways to limit brain injury after intracerebral hemorrhage, cerebral ischemia, and Parkinson's disease, and benefit public health.